Low-speed, high-torque motors have a wide range of applications as direct drive (DD) motors no reduction gear interposed. One example of use is a source for driving the arm of a robot. The motor described in the Japanese Patent Laid-Open No. 63974/1984 published on April 11, 1984 in the title of "Motor for High-torque robot", also belongs to this field of art.
A motor of this kind has teeth on the rotor and stator and produces a very large torque by arranging the stator along the inner and outer circumferences of the rotor.
By referring to FIGS. 3 through 5, the low-speed high-torque motor structure will be explained below.
Designated 1 is a rotor which has a magnet 2 that is magnetized in the axial direction. The rotor 1 has at each end a yoke 3 which has teeth formed on its outer circumference.
The flux going out of the magnet 2, as shown by the arrow in FIG. 3, passes the yoke 3 and the gap between the rotor and the stator and enters the stator. It then passes through the gap again and the rotor yoke 3 on the other side and then returns to the magnet 2. Many teeth are formed on the outer circumference of the rotor and on the inner circumference of the stator. The rotor has 90 teeth at even pitches and the stator has 10 teeth in each magnetic pole. There are eight magnetic poles that are arranged at equal pitches. The rotor teeth are spaced 4 degrees from each other and the stator poles are arranged at 45-degree intervals with 10 teeth on each pole spaced 4 degrees from each other.
The gap between the rotor and the stator is made as small as 70 to 100 .mu.m to increase the flux density.
Since in the prior low-speed high-torque motor in this field the gap between the rotor and the stator is made very small, a cogging torque results, producing variations in motor rotation. Unless the cogging torque is removed, a smooth rotation cannot be expected.
The cogging torque is variations in torque produced in the motor when the rotor in a deenergized state is rotated by external force. The cogging torque results from variations in magnetic flux which in turn are caused by variations in permeance of the gap between the rotor and the stator as the motor rotates. Factors that cause
permeance variations include errors in the tooth pitch of the stator and rotor and in the tooth width, stator pole pitch errors, deviation of centers of the stator and the rotor, and deflection of the rotor in operation. The tooth pitch errors of the stator and the rotor, the tooth width errors and the stator pole pitch errors depend on the accuracy of the pressing patterns since these components are pressed in the patterns. There are few variations among individual components. The center deviation and deflection, however, are caused by factors involving the assembly process and there are variations in error among individual components. As mentioned above, since the gap in the motor of this type is very small, even a slight center deviation or deflection will result in greater torque variations than in other types of motors. In an experimental motor, the torque produced varies from 200 gf-cm to 600 gf-cm, the largest being three times as large as the smallest.
In the low-speed high-torque motor, even slight torque ripples and cogging torques present a problem. In addition, this type of motor has a problem of characteristic variations among individual motors.
When there is no center deviation or deflection, the magnetic flux variations in each pole are sinusoidal as shown in FIG. 2 if we neglect the tooth pitch error and hysteresis saturation of the core material. The two magnetic fluxes of opposing phases cancel each other making the total flux at every angle constant. However, the actual products have manufacturing errors and bearing clearances, so that the center deviation or deflection cannot be eliminated.
On the other hand, the permeance is not proportional to the gap difference because the flux extends not only from the opposing tooth surfaces but from the sides of the teeth. Therefore, in a motor with a center deviation and deflection, the flux variations of each phase differ from those of the other phase for both the DC and AC components, so that these fluxes cannot cancel each other producing a cogging torque. The amounts of center deviation and of deflection vary from one product to another and their directions (phase) also differ, making it impossible to cancel the flux variations by a controlling means. Thus, solving the above problem by structural improvements has been called for.